Drilling mud introduced into an oil or natural gas well forms a filter cake on the drilling formations in the well. The filter cake helps to control the well, for instance, by providing sufficient hydrostatic head to overcome the reservoir pressure exerted by the reservoir of oil or natural gas accessed by the well. For high pressure formations, heavy drilling mud that contains high density solids can be used. For instance, barite can be used as a weighting material in heavy mud introduced into high pressure formations.